This invention relates to seismic roll along switching equipment and in particular to an improved switching system having a greatly reduced number of signal switches arranged to provide selectable gapping with maximum utilization of processing channels.
The roll along method of seismic geophysical prospecting is well known and commonly employed. In this method a large number of geophone groups and shot points are laid out along a prospecting traverse. During any given initiation, or shot, only part of the geophones are used to record reflections from that initiation. Thus for example, while one hundred and twenty geophone groups may be laid out along the traverse, only sixty may be active for a particular shot. A gap is usually provided about the shot point by leaving a number of groups immediately adjacent the shot point inactive. The active groups are usually spaced symmetrically about the gap. The active groups are those which are connected for a particular shot to a signal processing and recording channel. System capacity is usually defined by the number of signal processing channels which comprise filters and amplifiers matched to very high precision and account for the greatest portion of the systems cost.
Rather complicated roll along switches using either mechanical relays or sliding switches having thousands of switch contacts are commonly used for connecting appropriate geophone groups to processing channels. Commercially available switches using sliding contacts include a rotary version sold under the trademark "Rot-A-Long" by Inputs/Outputs Devices, Inc., of Houston, Tex. and a linear version sold under part number S100-002131 by GUS Manufacturing, Inc., of El Paso, Tex. Such switches are limited in several ways by the fact that they are basically mechanical devices. The thousands of switch contacts are prone to contact failure especially under the field conditions encountered in geophysical prospecting. The devices also tend to be inflexible in terms of the allowable gapping choices since the large number of switches must be arranged to provide at least one common set of roll along positions and it is difficult to modify that set to any great extent. While solenoid and ratchet arrangements can be used to remotely move the mechanical type switches through their various positions, any failure of the mechanism to operate typically causes erroneous selection of groups throughout the rest of the operation. As a result of these problems, roll along matrices have essentially always been housed in the main recording equipment near an operator who can visually check for proper operation.
One reason for the complexity of known roll along matrices is the usual requirement that processing channel outputs be in a particular order corresponding to the order of active channels. That is, if the geophone groups are numbered from one end of the traverse to the other, it is usually required that the lowest numbered active geophone group be connected to the first processing channel and the highest numbered group be connected to the last processing channel. The outputs of the processing channels are usually coupled by a multiplexer to a single analog to digital converter in a sequence beginning with the first processing channel and ending with the last. The multiplexer sequentially couples each channel to the A to D converter during each sampling period and the output of the converter is coupled to a recording system which therefore records the digitized samples always in an order beginning with the lowest numbered active group. Due to this ordering requirement, each of the processing channel inputs must be moved to a new group for each new shot. While such switching matrices are quite complicated and subject to contact failure, they do guarantee that at each shot point, each of the processing channels is utilized.
Many of the problems of the known roll along switching matrices are solved by the simplified roll along switching arrangement disclosed in application, Ser. No. 055,632 filed by Kamal A. Mahmood, concurrently herewith. In a simple form Mahmood's system provides one switch for each geophone input with one contact of each switch connected to one of the inputs and the other contact of each switch connected in sequentially repeating order to inputs of signal processing channels. As a result, the number of switches is reduced to the number of geophone groups and is only a small fraction of the number of switch contacts conventionally employed. Mahmood's system further provides a modified multiplexer addressing scheme for coupling signals to an analog to digital converter and recording system in the desired sequence even when the switches do not actually connect geophone groups to processing channels in that sequence. Any desired gap may be employed in Mahmood's system but for every inactive group, one of the signal processing channels is also inactive for the particular shot. For most general work, a selectable gap of from zero to fifteen groups is preferred. In the system taught by Mahmood, an additional fifteen processing channels must be provided if the full recording capacity of the system is to be employed for every shot and gap selection from zero to fifteen groups is also to be provided. As noted above, the signal processing channels are the single most expensive part of the system and the fifteen additional groups would significantly increase the price of most systems.
Thus, it can be seen that a reduction in the number of switches involved in roll along matrix as compared to the prior art mechanical devices is desirable and any substantial reduction in number would greatly enhance the accurancy and reliability of geophysical prospecting systems. It is also desirable that a roll along switching system provide a selectable gap while maintaining maximum utilization of processing channels. In addition, it is desirable that the switches employed in such roll along matrices be electronically switchable to avoid mechanical failures and increase flexibility of operation.